The present invention concerns circuits used for communication systems and pertains specifically to a duplexer with a differential receiver port implemented using acoustic resonator elements.
For applications such as cellular phones, it is desirable to reduce the size of components. Particularly, it is desirable to integrate RF duplexers and filters as part of a radio-on-a-chip with a readily manufactured technology.
Acoustic resonator elements have been used to implement filters. One advantage of using acoustic resonator elements is that the speed of sound is approximately three or four orders of magnitude smaller than the speed of light, making the wavelengths, and thus the dimensions of a device, small compared with conventional (L-C) tank circuits.
Currently available duplexers using acoustic resonators such as surface acoustic wave (SAW) elements or film bulk acoustic resonator (FBAR) are fully single ended. Such duplexers generally require that the receive (Rx) filter present a short circuit at the transmit (Tx) band frequency. This short circuit is transformed into an open circuit through the utilization of a quarter wave transmission line. The short circuit, in practice, is more capacitive than desired, and the quarter wave transmission line can be modified to account for this imperfection. The end result is still an open circuit at the transmit band frequency. This prevents the receive filter from loading the transmit path. Similarly, the transmit filter presents an open circuit at the receive band frequency, directly. This is accomplished by starting with a series resonator.